(Portions of the technical material contained in this section may not be prior art.)
State of the art radio frequency (RF) electrical circuits use large quantities of passive devices. Many of these circuits are used in hand held wireless products. Accordingly, miniaturization of passive devices and passive device circuits is an important goal in RF device technology.
Integration and miniaturization of passive devices on the scale of active silicon devices has not occurred for at least two reasons. One, typical passive devices to date employ different material technologies. But, more fundamentally, the size of many passive devices is a function of the frequency of the device, and thus is inherently relatively large. However, still, there is unrelenting pressure to produce more compact and area efficient IPDs.
Significant advances have been achieved. In many cases these involve surface mount technology (SMT). Small substrates containing large numbers of passive components are routinely produced using surface mount technology.
More recent advances in producing integrated passive device networks involve thin film technology where resistors capacitors and inductors are built as integrated thin film devices on a suitable substrate. See for example U.S. Pat. No. 6,388,290, incorporated herein by reference. This approach is being used widely in advanced technology products.
With increasing miniaturization, and continuing shrinking of IPD dimensions and features, electrical interactions between the IPD substrate and the passive devices mounted on the substrate are of growing concern. U.S. patent application Ser. No. 10/277,239, filed Oct. 21, 2002 addresses these issues, and describes and claims a high resistivity IPD substrate that offers processing advantages coupled with the desired dielectric properties. This substrate can also be made thin, to reduce the profile of the IPD. The substrate described in the aforementioned application is intrinsic silicon, with an oxide layer on the surface of the silicon. When the oxide layer is made thin, as in the preferred embodiments described in that application, charge build-up occurs in the interface between the high resistivity substrate and the oxide layer. The combination of the high resistivity substrate and the oxide layer behaves as a so-called dual dielectric, a structure well known for creatively employing charge storage effects. The charge accumulates at the interface, and the electric field produced by the accumulated charge influences both the substrate characteristics and the characteristics of electrical devices on the substrate.
If the accumulated charge is substantial, the silicon substrate appears as if semiconducting, creating an MOS structure. When the surface mounted IPD device is a capacitor for example, the MOS structure acts as an added series capacitor, and degrades the capacitor performance. Moreover, if the accumulated charge renders the substrate semiconducting, the performance of all of the IPD components on the surface is impaired due to the reduced resistivity of the entire substrate.
A significant part of these adverse effects on passive components in the IPD device is due to the fact that much of the accumulated charge just described is mobile. Due to the mobility of the accumulated charge, the adverse electric field effects from the accumulated charge varies with applied voltage. Consequently, while the occurrence of charge states in the silicon/silicon oxide interface is difficult to avoid, the adverse field effects of the accumulated charge may be reduced by fixing the charge in charge traps in the silicon.
One approach to creating charge traps and fixing the mobile charge at the interface is described by Janseman et al., “Elimination of accumulation charge effects for High-Resistivity Silicon Substrates” [reference] Janseman et al. create ion implantation damage in a surface layer of the silicon substrate. It is well known that ion implantation damage reduces charge mobility in silicon due to the creation of crystal damage and the production of charge trapping sites.
Other approaches that improve the surface characteristics at the silicon/silicon oxide interface would contribute new dimensions to IPD technology.